Track error correction system as for video disc player

ABSTRACT

In a system for correcting tracking errors in a video disc player, digital information, corresponding to field numbers is encoded on the video signal during one horizontal line of the res ective vertical blanking interval. Tracking errors, such as a locked track condition or a skipped track condition are detected by noting improper sequence of two or more decoded field numbers. A kicker apparatus is provided to move the signal pickup device in a direction so as to reduce the tracking error.

This invention relates to systems for detecting tracking errors in avideo disc player.

A video disc is a flat body member having a signal track on the surfacethereof. The signal tracks are often very fine so that the disc maycontain an adequate length of program material and provide sufficientbandwidth for a video signal. In most video disc systems, the video discgenerally includes some structure between signal tracks which allows apickup apparatus to follow the information track as the disc rotates. Ina grooveless, capacitive pickup system, left and right marking signalsare recorded between adjacent signal tracks to guide the pickup servosystem along the center of the signal track. In some optical pickupsystems, the vacant space between spiral signal tracks is used by theoptical servo system for tracking. In some grooved disc systems, theraised side walls of the groove on the record provide mechanical forcesto guide the pickup stylus during playback.

For each type of video disc and its particular servo system there is aclass of disc defects and contaminants which will cause the pickupapparatus to jump over one or more signal tracks, resulting in atracking error. In the grooved system discussed herein a caddy is usedto protect the disc from contaminants and mechanical damage. However, inuse, it is always possible that contaminants will enter the protectivecaddy, attach to the disc, and cause tracking errors.

Tracking errors can occur in either direction in the program material. Abackward skip of the pickup will cause a repeat traversal of one or morepreviously traversed convolutions of a spiral signal track. Suchcondition is referred to herein as a "locked groove" condition. Even ifthe pickup eventually breaks out of the locked groove, programinterruption is significant. A forward skip, while unobjectionable forsome video programs, is a serious problem for certain types of programmaterial recorded on the video disc. It is therefore highly desirable toavoid tracking errors of both the forward skip and locked groove types.

As mentioned above, tracking errors may result from defects in therecords during manufacture or from contaminants such as dust particlesand fingerprints adhering to the surface of the record during normaluse. The signal track density on long playing video discs is generallyvery high e.g. almost 10,000 grooves per inch in some cases, and thus itis difficult to develop manufacturing techniques where by all defectsare eliminated. Even when a protective caddy is used after manufacture,it is difficult to completely protect the disc from environmentalfactors.

These facts tend to mandate that one develop a system for detecting andcorrecting tracking errors. Such a system permits the use of video discscontaining some manufacturing imperfections. For example, a video discwhich contained a minor locked groove, which would otherwise cause aprogram interruption, could then be successfully played on a playerusing the present invention. Further, a video disc which, in use,develops several locked grooves due to contaminants such as dustparticles, would have its useful life extended accordingly.

One prior art scheme for detecting tracking errors on a grooved videodisc record is to record an audio signal, 18 KHz for example, on thevideo disc. A tracking error is then detected as a phase shift in thevideo tone. The direction of the phase shift, whether lead or lag,indicates the polarity of the tracking error. One problem with the aboveapproach is that its range is limited, i.e. a leading phase shift of180°, or more, cannot be distinguished from a lagging phase shift.Another problem is that cross modulation products with other recordedsignals, such as the 15 KHz line frequency, tend to produce an audiblebeat frequency.

An approach using the 15 KHz line frequency itself for detectingtracking errors is found in U.S. Pat. No. 3,963,860, "LOCKED GROOVEDETECTION AND CORRECTION IN VIDEO DISK PLAYBACK APPARATUS" issued June15, 1976 to T. W. Burrus. In the Burrus patent, each convolution of thespiral signal track contains a fixed number of horizontal lines plus afraction, say 0.1, or a horizontal line. The horizontal sync pulses,therefore, are spirally aligned, rather than radially aligned. Atracking error is detected by noting a consistent phase shift in thehorizontal sync pulses. The "spiral sync" approach also suffers fromlimited range in that a skip forward of 5 grooves (i.e. a phase shift of0.5 of a horizontal line in the above example) cannot be distinguishedfrom a skip backward of 5 grooves. Also, track error detection by phaseshift detection becomes complex when transient effects during stylusset-down are taken into account. Furthermore, the detector mustdiscriminate against noise effects to prevent false locked grooveindications which would otherwise advance the stylus unnecessarily.

In the present invention, tracking errors are detected by sensingdigital numbers, prerecorded on the video disc in a prescribedsequence--e.g., each digital number increasing incrementally over thepreceding one, wherein a tracking error is detected by noting thedeparture of two sequentially sensed digital numbers from orderlysequential relationship. Since the direction and magnitude of trackingerror is known from the difference between the two sensed digitalnumbers, a kicker means can be operated in such direction and accordingto such suitable control algorithm as to pulse or steer the stylus backto the desired track. A copending patent application "VIDEO DISCSYSTEM", by Charles Dieterich, filed concurrently herewith and assignedto the assignee of the present invention, describes a digital datasystem for use with the present invention.

Once a tracking error is detected, the regular pickup servo system maybe activated to move the pickup to the correct track. However, theregular pickup servo, while suitable for tracking the gradual advance ofthe stylus along the spiral signal track, may not have the necessaryhigh frequency response to correct a tracking error. Therefore, aseparate kicker means is provided to produce local rapid movements ofthe pickup to adjacent tracks. Such kicker means can be electromagnetic,piezoelectric, or any device for producing appropriate mechanical motionof the pickup.

IN THE DRAWING

FIG. 1 is a graphical representation of a television signal includingthe vertical blanking interval between odd and even fields;

FIG. 2 is a graphical representation of the digital data format used inthe recording method of the present invention;

FIG. 3 is a block diagram of a video disc encoder in accordance with thepresent invention;

FIG. 4 is a block diagram of a video disc player embodying the presentinvention;

FIG. 5 is a block diagram showing more detail of the digital datagenerator of the video disc encoder of FIG. 3;

FIG. 6 is a block diagram showing more detail of the information bufferfor the video disc player of FIG. 4;

FIG. 7 is a schematic diagram of a means for generating an error checkcode from the information bits for the video disc encoder of FIG. 5;

FIG. 8 is a schematic diagram, shown partially in block form, of theinformation buffer for the video disc player of FIG. 4;

FIG. 9 is an embodiment of a receiver control counter for theinformation buffer shown in FIG. 8;

FIG. 10 is a state transition diagram for the microprocessor controlmeans of FIG. 4; and

FIG. 11 is a flow chart representing a program algorithm for themicroprocessor control means of FIG. 4;

SIGNAL FORMAT

Particular details of an NTSC type television signal formatted inaccordance with the buried subcarrier technique as described in U.S.Pat. No. 3,872,498, "Color information translating systems", to D.Pritchard, are shown in FIG. 1. A vertical blanking interval separatesthe interlaced odd and even fields. Those skilled in the television artswill readily recognize the standard vertical blanking intervalcontaining a first equalizing pulse interval, a vertical sync interval,a second equalizing pulse interval, followed by a number of horizontalline intervals at the start of each new field. As shown in FIG. 1, thevideo signal information begins on line 22' of field 1, and on line 284'of field 2.

The digital information representative of the field number appears atline 17' of field 1, and line 280' of field 2. Digital informationcould, as well, be inserted in other lines of the vertical blankinginterval. To show the details of the digital signal format, FIG. 2expands the time scale during the horizontal line containing data (line17' or line 280').

Data are represented in terms of luminance level: 100 IRE units is alogical "one" and 0 IRE units (blank) is a logical "zero". The firstdata bit follows the standard horizontal sync pulse 140 and color burst142. The frequency of the burst 142 is about 1.53 MHz, the frequency ofthe buried subcarrier. Each data bit is transmitted synchronously withthe 1.53 MHz buried subcarrier signal. As shown in FIG. 2, the digitalmessage comprises a 13-bit start code termed B(x), a 13-bit redundanterror check code termed C(x), and 51 information bits termed I(x). Thebeginning of the next horizontal line is indicated by the nexthorizontal sync pulse 140a and color burst 142a. Thus, the individualdata bits are synchronous with the color subcarrier, and the overalldigital message is synchronous with the vertical sync pulse. Note thatthe data rate can be a multiple or submultiple of any convenientsubcarrier frequency. Also, other values of luminance may be assigned tologic one and zero, or more than one bit may be associated with a givenluminance level.

A start code is used in the present system to synchronize the datasystem to the digital message thereby avoiding the need to detect theedge of horizontal or vertical sync. Synchronizing errors in a serialdigital data system result in framing errors, i.e. where received datais shifted by one or more bits from its proper position. Previouslyknown systems for recording digital data on a video disc encoded signalhave shown that the edges of sync signals are not reliable as a timereference and have resulted in framing errors. Start codes have provento be more reliable.

The specific start code chosen, 1111100110101, is one of the Barkercodes known in radar and sonar technology. See "Group Synchronization ofBinary Digital Systems", by R. H. Barker, published 1953 by AcademicPress, New York, N.Y. Barker codes are designed such that theauto-correlation function, of a signal containing a Barker code shiftedwith respect to itself, is maximized when coincidence occurs, andminimized elsewhere. That is, if one assigns a value of +1 or -1 to eachbit in the start code and computes the sum of the respective bitproducts for each shifted position of the start code with respect toitself, such auto-correlation function will produce a sharp maximum whencoincidence occurs. Specifically, a Barker code shifted any odd numberof places with respect to itself produces an auto-correlation of 0. ABarker code shifted any even number of places with respect to itselfproduces an auto-correlation of -1. However, when there is coincidence,the auto-correlation is N, where N is the number of bits in the Barkercode. In other words, a Barker code shifted any number of places withrespect to itself differs in a maximum number of bit positions. In thepresence of noise, this characteristic reduces the probability of afalse start code detection, as compared to an arbitrarily chosen startcode.

The information bits, I(x), include a field number, a band number, andspare information bits for future expansion. Field numbers identify eachfield of the video signal by a unique 18-bit binary number. At thebeginning of the video disc, the first field of the video program isfield "zero". Thereafter, each field is consecutively numbered inascending order. Band numbers refer to recorded video signal in a groupof adjacent convolutions of the spiral grooves which form a band-likeshape. All of the material in such band of grooves is identified byhaving a common band number. As an example of band number utility, thevideo signal after the end of the video program material is recordedhaving band number "sixty-three". The video disc player senses bandsixty-three as the end of program and responds by lifting the stylusfrom the record.

The error check code C(x) is computed from I(x) in the video discrecording apparatus. To this end, I(x) is multiplied by a constant,H(x). The resulting product is divided by another constant g(x). Aftersuch division, the remainder (the quotient is unused) is added to athird constant M(x). The result is C(x).

In the video disc player, the received message is checked for errors bydividing the entire message, including the start code, by the constantg(x) mentioned above. If the remainder is equal to the start code, B(x),then the message is considered error free. The constants H(x) and M(x)are chosen so that the remainder of the entire message will in fact bethe start code. The constant g(x), used in both the video disc recordingapparatus and the video disc player is called the generator polynomialof the code. A specific g(x) is chosen which generates a code havingerror detection properties particularly advantageous as applied to thevideo disc medium. In the system described herein, the addition,multiplication, and division operations referred to above are performedaccording to special rules to accomodate the hardware available forcarrying them out. The error coding will be discussed in greater detailhereinafter in conjunction with the encoding and decoding hardware.

A block diagram of a video disc encoder is shown in FIG. 3. A compositevideo signal from source 30 is linearly combined in adder 36 with adigital data bit stream on conductor 37 supplied by the digital datagenerator 38. Synchronizing means 32 supplies a color subcarrier andsynchronizing pulses so that the data bits generated by the digital datagenerator 38 are synchronous with the color subcarrier appearing atterminal 31a and so that the digital message is encoded on the properhorizontal line in the vertical blanking interval. Information bits,appearing at data bus 39 and representing the video field number andband number, are provided by apparatus 34. The use of field number andband number information will be discussed in conjunction with themicroprocessor program (FIG. 10 and 11). The digital data and the videosignal are combined in the adder 36. Further signal processing means 40conditions the composite video for the recording medium. The compositevideo signal is of the buried subcarrier type and is recorded using FMmodulation techniques.

In the video disc player of FIG. 4, the FM signal is detected usingpickup transducer and stylus assembly 20 and converted in videoprocessing circuitry 18 to a standard television signal for viewing onan ordinary television receiver. Video processing circuitry 18 includesmeans responsive to the color burst signal to phase lock a 1.53 MHzlocal color oscillator to the color subcarrier. The color oscillator, inaddition to its usual use for demodulating the buried subcarrier wave,is also used to provide the digital clock signal and this signal appearson conductor 72. The video processing circuitry 18 further includesmeans for demodulating the video carrier and comb filtering therecovered video signal. Comb filter 19 subtracts two adjacent fieldlines, which result appears on conductor 70 as processed video. Sinceline 16', which is at the black level, is subtracted from line 17',which is modulated with digital data, the processed video on conductor70 is the recovered digital data. Naturally, line 16' may be anyconstant luminance level. Note that if the subsequent line 18' to thedata line 17' is a constant luminance line (also black) the subsequentoutput of the comb filter during line 18' is again recovered digitaldata, but the data is inverted. By subtracting one line from a constantluminance adjacent line, the recovered digital signal isself-referenced, thereby eliminating data errors due to shifts in thed-c level of the video signal. If it were desired to place data onconsecutive lines, as compared to placing data adjacent to constantluminance lines, then means for referencing the video signal to apredetermined luminance level or a d-c reference level would benecessary in order to separate the digital data stream from the videosignal.

As shown in FIG. 4, the information buffer 16 is responsive to processedvideo on conductor 70 and the 1.53 MHz clock signal on conductor 72 toextract digital data from the video signal. The buffer 16 is controlledby a digital binary control signal on conductor 71 from themicroprocessor 10. In one binary state, the control signal on conductor71 causes the information buffer 16 to acquire data. In the other binarystate, the control signal on conductor 71 conditions the informationbuffer 16 to transfer the received data to the microprocessor 10. Inparticular, when the control signal on conductor 71 is high, theinformation buffer 16 opens to sample incoming data on the processedvideo signal conductor 70 using the 1.53 MHz signal on conductor 72 as aclock. After a complete message is received, the status signal onconductor 75 furnishes an indication that a message is complete. Totransfer the message to the microprocessor memory, the control signal onconductor 71 is set low. This action closes the information buffer 16,resets the internal control circuits, and gates the results of themessage error code check onto status conductor 75. If the status signalindicates the message is valid (i.e. error code check indicatesvalidity), the microprocessor 10 is programmed to transfer the data inthe information buffer 16 to the microprocessor 10. The microprocessorsupplies an external clock signal on conductor 73 to transfer data fromthe information buffer 16. For each clock pulse, one bit of data onconductor 74 is shifted out of the information buffer and into themicroprocessor 10. When all the data is transferred to themicroprocessor 10, and the program is ready for another digital message,control conductor 71 is again returned to a high state and the processis repeated.

The microprocessor 10, via the information buffer 16, controls thegating of line 17' (or line 280') out of the video signal. The firstdigital message is obtained by continuously searching the video signalfor a start code. Thereafter, the information buffer 16 is closed. Then,based on the time of arrival of the first digital message, theinformation buffer is opened approximately six lines before the nextdigital message is expected. If no valid message is found, theinformation buffer 16 is closed approximately six lines after suchexpected time of arrival. If a valid digital message is found, theinformation buffer 16 is closed and a new time of arrival for the nextdigital message is calculated based on the time of arrival of thecurrent digital message. In such manner, the microprocessor 10 opens agate, or "data window", approximately twelve lines wide and centeredabout the expected data. The time interval from the center of one datawindow to the next is approximately one video field time interval. Thewidth of the data window is chosen so that under worst case timingconditions the expected data will fall within the data window. Sourcesof timing error, as explained below, are: finite resolution of thedigital timer; the drift rate of the timer; program uncertainty indetermining time of arrival of present data; and timing differencesbetween odd and even interlaced fields. Use of an alternatemicroprocessor and/or timer may be accommodated by adjusting the datawindow width accordingly. The microprocessor program which controls thelogic for searching for data and centering the data window is discussedhereinafter in conjunction with FIGS. 10 and 11.

The microprocessor 10 is also responsive to the player panel controls 14(load, pause, and scan) to operate the player mechanism 12 and drive theplayer display 22 in accordance with a predetermined program, asdiscussed hereinafter. The player mechanism is further provided with atleast one stylus "kicker" operable by the microprocessor 10. A kicker isa means, piezoelectric, electromagnetic, or otherwise, for impulsivelymoving the signal pickup means to adjacent grooves or signal tracks onthe video disc medium. The use of the kicker to break out of lockedgrooves will be discussed hereinafter in conjunction with the flowdiagrams of FIGS. 10 and 11.

ERROR CODE

As mentioned above, the video disc recording apparatus uses theinformation bits I(x) to compute C(x). Because of the large number ofpotential combinations--I(x) and C(x) together are 64 bits long--and thedesire to determine the error detection and correction characteristicsof a given code without resorting to enumeration, error codes aretreated mathematically. A general mathematical development of ringtheory and Galois Fields GF(2^(m)), applicable to error codes ingeneral, can be found in "Error Correcting Codes" by W. Wesley Peterson,published by MIT Press, Cambridge, Massachusetts. For present purposes,the error coding in the video disc may be best understood in terms of afew simple definitions.

A digital message, comprising ones and zeros, can be considered asrepresenting an algebraic polynomial comprising powers of x. Thecoefficients of the respective powers of x are the individual bits ofthe message. For example, the 4 bit message 1011, can be represented bythe polynomial P(x), where ##EQU1##

Applying this notation to the start code, 1111100110101, then

    B(x)=x.sup.12 +x.sup.11 +x.sup.10 +x.sup.9 +x.sup.8 +x.sup.5 +x.sup.4 +x.sup.2 +1

The highest power of x is called the degree of the polynomial. In theabove example, B(x) is a polynomial of degree 12.

Polynomials may be added, subtracted, multiplied, and divided using theordinary rules of algebra except for expressing coeficients in modulo 2terms. A shorthand notation for the remainder of a polynomial afterdivision by another polynomial is indicated by brackets. That is, if##EQU2## where the remainder, r(x), has a degree less than the divisor,g(x), then

    [P(x)]=r(x)

In the video disc recording apparatus, the total message recorded on thevideo disc is represented by a polynomial, T(x). From FIG. 2,

    T(x)=B(x)x.sup.64 +C(x)x.sup.51 +I(x)                      (1)

The term x⁶⁴ shifts B(x) by 64 bits, because B(x) is at the beginning ofthe data format. Similarly, the term x⁵¹ shifts C(x) 51 bits torepresent that C(x) is recorded before I(x). In accordance with theapparatus being described, the recording apparatus computes a value forC(x) so that the total message, T(x), has a remainder equal to B(x)after being divided by g(x). That is, assuming C(x) to be of the form

    C(x)=[I(x)·H(x)]+M(x),                            (2)

then H(x) and M(x) are constant polynomials chosen so that

    [T(x)]=B(x)                                                (3)

It can be shown that equations (1), (2), and (3), when solved for theconstant polynomials H(x) and M(x), yield

    H(x)=[x.sup.127 ]

    M(x)=[B(x)x.sup.13 +B(x)x.sup.127 ]

FIG. 7 includes a table enumerating the chosen values for B(x) and g(x),as well as the derived values for H(x) and M(x). Note that the table inFIG. 7 shows high order bits on the right, so that they are in the sameorder as the flip flop storage elements appear in the logic diagram ofthe same figure.

In the video disc player, the recorded digital message is read by theplayer electronics. The data recorded on the video disc is T(x). Thedata read by the player is R(x). If no errors are generated betweenrecording and playback then T(x)=R(x). The received message, R(x), ischecked for errors by dividing R(x) by g(x). If the remainder is equalto B(x), the start code, then the message is considered error-free. Onthe other hand, if the remainder does not equal B(x), then an error isthereby indicated.

The characteristics of a code generated in the above manner depend onthe choice of g(x), which is called the generator polynomial. Theparticular g(x) chosen for the video disc medium is one from thecomputer generated codes demonstrated by Tadao Kasami in "OptimumShortened Cyclic Codes for Burst Error Correction" published in IEEETransations on Information Theory 1963. A burst error in a digitalsystem is a type of error where adjacent bits in the digital message arelost. Burst errors are considered a likely type of transmission error inthe video disc medium. As shown by Kasami in the aforementionedreference, a code which can correct single burst errors of 6 bits orless, can be implemented using a generator polynomial given by

    g(x)=x.sup.13 +x.sup.12 +x.sup.11 +x.sup.10 +x.sup.7 +x.sup.6 +x.sup.5 +x.sup.4 +x.sup.2 +1

Furthermore, it may be shown that for the g(x) given above, all singleburst errors of 13 bits or less will be detected, and 99.988 percent ofall single burst errors longer than 13 bits will be detected as well.The video disc player, as described herein, uses only the errordetection capabilities of the chosen code.

As a specific example of error code generation, consider the case wherethe field number is 25,000, the band number is 17, and the spare bitsare 0. Since 25,000 in binary representation is 000 110 000 110 101 000,and 17 in binary representation is 010 001 (high order bits are on theleft), the 51 information bits are 000 000 000 000 000 000 000 000 000000 110 000 110 101 000 010 001. The order of transmission is spare bitsfirst, followed by field number, and then band number, wherein the mostsignificant bit is transmitted first. The error code for the abovespecific I(x), computed as the remainder of I(x) times H(x), plus M(x),is represented by 0111100100010. The next video field is 25,001 or 000110 000 110 101 001 in binary representation. For the correspondinginformation bits, 000 000 000 000 000 000 000 000 000 000 110 000 110101 001 010 001, the proper error code is 1000101101110. The completedigital message for field 25,001 including the start code is therefore,1111100110101 1000101101110 000 000 000 000 000 000 000 000 000 000 110000 110 101 001, 010 001, shown in order of transmission. The start codeis the first 13 bits, the error code is the next 13 bits, and the 51information bits are last. In the video disc player, the above digitalmessage is checked for errors by dividing the received message by g(x).If no errors are detected, the remainder is 1111100110101, which isexactly the start code.

HARDWARE

A block diagram of a means for generating T(x) is shown in FIG. 5. Underthe control of the transmitter control means 50, 24 information bits areloaded via data bus 39, and 27 spare information bits are loaded viadata bus 39a into a 51 bit shift register 44. I(x), which comprisesthese 51 bits, is then shifted into another 51 bit shift register 52.

At the same time, during the 51 shift pulses, an encoder 45 computesC(x) in the following way. Polynomial dividing and multiplying means 46is responsive to the 51 bit serial transmission of I(x) to compute theremainder of I(x) times H(x) divided by g(x). M(x) is then added inparallel in polynomial adder 48. The resulting code C(x) is loaded intoa 13-bit shift register 54, and B(x), the start code, is loaded via databus 49 into another 13 bit shift register 47. Since the start code is aconstant digital value, such loading is preferably accomplished by fixedconnections to the parallel load inputs of shift register 47 as opposedto a software implementation. In positive logic notation, thecorresponding parallel inputs to shift register 47 are connected toground potential wherever the start code has a zero, and to a positivepotential wherever the start code has a one. Transmitter control means50 controls the total message T(x), contained in the three shiftregisters 52, 54, 47, being shifted out serially in synchronism with thecolor subcarrier on conductor 31a. A video synchronizing pulse appliedon conductor 33 provides transmitter control means 50 with a timereference so that the digital message is transmitted at the proper timewith respect to the video signal.

A specific embodiment of the encoder (45 of FIG. 5) is shown in FIG. 7.Clocked flip flops having output terminals Q₀ through Q₁₂ form aremainder register. Multiplication by H(x) and division by g(x) isperformed simultaneously in bit serial fashion. Afterwards, theremainder is held in the remainder register outputs Q₀ through Q₁₂. SeeChapter 7, pages 107-114 of the above-mentioned Peterson reference for ageneral treatment of such circuits. To appreciate the simplicity of thecircuit in FIG. 7 for multiplying and dividing polynomials, it is notedthat both addition and subtraction (of coefficients of terms of likepower) is performed by an exclusive OR gate. Multiplication of I(x) byH(x) is performed by appropriate connections to one or more exclusive ORgates 80 through 91. In particular, wherever a coefficient of H(x), butnot g(x), is equal to 1, (bit positions 1, 3, and 8) input I(x) isconnected to an input of an exclusive OR gate 80, 82, and 87,respectively. Division of I(x) by g(x) is performed by multiplying theoutput of Q₁₂ by g(x), and subtracting the resulting product from thecontents of register Q₀ through Q₁₂. In particular, wherever acoefficient of g(x), but not H(x), is equal to 1, (bit positions 4, 7and 11) the output of Q₁₂ is connected to an input of exclusive OR gate83, 86, and 89, respectively. Where H(x) and g(x) are both equal to 1(bit positions 0, 2, 5, 6, 10 and 12) the output of exclusive OR gate 91is connected to an input of exclusive OR gates 81, 84, 85, 88, and 90,respectively. After 51 clock pulses, one for each bit of I(x), thecontents of register Q₀ through Q₁₂ is the remainder of I(x)·H(x) afterdivision by g(x).

Now how M(x) is added to the contents of the remainder register.Addition of coefficients is in modulo 2 arithmetic performed as theexclusive OR function. Wherever M(x) has coefficients of +1, thecomplement output, Q, of the corresponding flip flop is used; whereverM(x) has coefficients of 0, the uncomplemented output, Q, is used.

A block diagram of a means for decoding the received message, R(x),appears in FIG. 6, which is an embodiment of the information buffer 16of FIG. 4, discussed above. Control signal on conductor 71, an input,conditions the receiver decoder of FIG. 6 either to receive data fromthe video signal, or transfer data to the microprocessor.

In the receive state, each bit is simultaneously shifted into twoseparate registers. One such register 60 is for data, and the other 62is for error checking. The error check register 62 is a polynomialdivider. However, when acquiring new data, the divider feedback path isdisabled so it functions as a straight shift register. The opertion ofdivider register 62 will be discussed subsequently in greater detail inconjunction with FIG. 8. For present purposes, register 62 is responsiveto the receiver control means 64 to either shift in successive bits ofR(x), or divide successive bits of R(x) by g(x). In either case, thecontents of register 62 is available on data bus 78 and provided to thestart code and valid data detector 66.

The receive operation begins with register 62 conditioned to operate asa shift register. After B(x) is detected by detector 66, control means64 conditions register 62 to operate as a polynomial divider. Thus,polynomial division by g(x) begins with B(x) in the divider register 62.The receiver control means 64 is further responsive to the detection ofB(x) to time out a period equal to the remaining message bits (64 clockpulses). After the time out period, the divider 62 contains theremainder of R(x) modulo g(x), which should be B(x) if the message isvalid. During the error check process, data register 60 has beenshifting in data bits. At the end of the time out period, the dataregister 60 stores only the last 24 bits. However, since the 24information bits are placed at the end of the message, register 60 willcontain the assigned information bits. If it is desired to utilize thespare information bits, additional shift register stages may be added.

Interpretation of the output status signal on conductor 75 depends onthe state of control signal on conductor 71. When the control signal onconductor 71 conditions the receiver for acquiring data (receive state)the status signal on conductor 75 is defined as "message received". Whenthe control signal on conductor 71 conditions the receiver fortransferring data (transfer state), status signal conductor 75 indicates"data valid". The control signal on conductor 71 also resets thereceiver control means 64 and gates the results of the remainder checkonto the status signal on conductor 75.

The received information is transferred out of shift register 60 inresponse to external clocks supplied by the microprocessor on conductor73. After the data is shifted out, the control signal on conductor 71may be returned to its previous state which will again condition thereceiver-decoder to continuously search for another start code.

FIG. 8 shows a logic diagram, partially in block form, of the receiverdecoder of FIG. 6. The flip flops having output terminals Q₀ ' throughQ₁₂ ' form a remainder register. Polynomial division by g(x) isperformed by multiplying sucessive register output terms from Q₁₂ ' byg(x) and subtracting the product (via exclusive OR gates 100 through108) from the contents of the remainder register. A feedback connectorfrom Q_(12') , (through NOR gate 109) is made to an exclusive OR gatewherever g(x) has coefficients of 1, except for bit 13. Since thecoefficients of g(x) are 1 for bit positions 0, 2, 4, 5, 6, 7, 10, 11,12, an exclusive OR gate is placed at the data input of each respectiveflip flop of the remainder register as shown. NAND gate 118 detectsB(x), which is both the start code and the valid error check code. Thereceiver control counter 117, begins counting responsive to a startsignal from AND gate 120, counts 63 clock periods and supplies a stopsignal which is used by NAND gate 111 to stop the clock to all decoderflip flops. A representative embodiment of the receiver control counter117 is shown in FIG. 9 comprising seven flip flops 130 through 136.

The sequence of operations in receiving data is as follows. When thecontrol signal on conductor 71 is high, data is gated to divider 62through AND gate 110. Flip flop 119 has been previously set, whichdisables the feedback signals in divider 62 by blocking NOR gate 109.Register 62 now functions as a shift register. Upon detecting B(x), theoutput of NAND gate 118 goes low, and the Q output of flip flop 119 goeslow one clock period later. Therefore, feedback is enabled forpolynomial division by the output of AND gate 120 via NOR gate 109 whenB(x) is detected in the remainder register. After 63 clock periods, thereceiver control counter 117 stops and the status signal on conductor 75goes high, indicating "message received". Shift register 60 holds thelast 24 bits of I(x). To transfer data, the control signal on conductor71 is made low. The inverted output of NAND gate 118, which is low ifthe remainder after division if B(x), is gated onto the status signal onconductor 75. External clock pulses on conductor 73 cause successiveshifts of data in register 60 to the output data signal on conductor 74.The external clock pulses also clear the remainder register by shiftingin zeros.

The preferred embodiment, above, shows a remainder register beginningand ending with the same non-zero constant. However, it will beunderstood that other arrangements are possible when using a coset code.For instance, after detecting B(x), the remainder register can be set toa first arbitrary constant. Then, after division, the remainder registeris checked for a proper second constant. The first constant, or thesecond constant, may be zero; both constants may not be zero.

Observe the simplified hardware which results from the error code formatdescribed herein. By ending with the start code, B(x), as a validremainder, the start code detector (NAND gate 118) also serves as avalid code detector. By beginning division with the start code in thedivider, a control step is eliminated in not having to clear theremainder register.

Typically, error codes are placed at the end of a message. In accordancewith the present invention, by placing the error code before theinformation bits, the receiver controller is further simplified in nothaving to distinguish information bits from error code bits with regardto the data storage register 60. In addition, the receiver controller,as shown in FIG. 8, is a simple counter 117, having a start terminal, astop terminal, and providing a timing out for a single time interval.

MICROPROCESSOR IMPLEMENTATION

Digital information, including band number and field number, arerecorded on the video signal, and utilized by the player to achieve avariety of features. Band number information is used by the player todetect end of play (band sixty three). Field number information inascending order is used to calculate and display the program playingtime on LED display means 22 in FIG. 1. If the length of programmaterial is known, field number information can be used to compute theremaining program playing time. For NTSC type signals, elapsed programtime in minutes may be obtained by performing the calculation ofdividing the field number by a constant number equal to the number offields per unit of playing time, e.g., by 3,600 for a disc formatted foran NTSC player. If desired the remaining program time may be derivedfrom the previous calculation. This feature is useful to the viewer whenscanning for a desired point in the program. A particularly usefulfeature, derived from field number information, is locked groovecorrection which will be discussed hereinafter in conjunction with themore general case, track error correction.

Field numbers represent actual stylus position. Thus, whenever thestylus re-enters a groove, whether after jumping tracks or after thescan mechanism is operated, the actual stylus position can be determinedfrom the first valid field number read. Both the track error correctionsystem and the program playing time display means use field number data,and therefore share the decoding portion of the video disc digital datasystem. The particular embodiment of track error correction systemdiscussed herein after uses field number data (stylus position) to keepthe stylus at or ahead of its expected position assuming a predeterminedstylus/record relative velocity. The program playing time display usesfield number data for an indication of playing time, which is actuallyanother representation of stylus position.

The microprocessor controller has several internal modes. FIG. 10 is astate transition diagram indicating the mode logic performed by themicroprocessor program. Each of the circles represent a machine mode:LOAD, SPINUP, ACQUIRE, PLAY, PAUSE, PAUSE LATCHED, and END. For eachmode, the position of the stylus and the status of the display isindicated inside each respective circle. The arrows between modesindicate the logical combination of signals, supplied by the panelcontrols (load, pause, scan), that cause a transition from one mode toanother. The load signal indicates that the player mechanism is in acondition to receive a video disc. The pause signal is derived from acorresponding control panel switch, and the scan signal indicates theoperation of the scan mechanism.

After power is turned on, the system goes into LOAD mode. A video discmay be loaded onto the turntable in this mode. After loading, the playerenters SPINUP mode for several seconds, allowing the turntable to bebrought up to the full speed of 450 RPM. At the end of SPINUP mode, theACQUIRE mode is entered.

In ACQUIRE mode, the digital subsystem lowers the stylus andcontinuously searches for a "good read". In ACQUIRE mode, a "good read"is defined as a valid start code and a valid error check remainder.After finding a "good read" the system enters PLAY mode.

In PLAY mode, the microprocessor establishes in memory an expected, orpredicted, next field number. The predicted field number is incrementedor updated each field reads, the microprocessor uses the predicted fieldnumber in performing two additional checks to further improve theintegrity of the data.

The first additional check is a sector check. The video disc in theembodiment under consideration contains eight fields in everyrevolution, dividing the disc into eight sectors. Since the relativephysical position of the sectors is fixed, the sectors follow a periodicrecurring order as the disc rotates, even if the stylus jumps over anumber of grooves. Although the digital information cannot be read forone or more fields, (sectors) while the stylus is skipping to a newgroove, the microprocessor keeps time, and increments the predictedfield number accordingly. When the stylus settles in a new groove andpicks up a new digital message, the new field number is checked bycomparison to the predicted field number. If the sector is wrong, thedata is considered a "bad read".

Field number is represented by an 18 bit binary number. Sectorinformation may be obtained from field number by finding the remainderafter dividing the field number by eight. However, it is noted that thethree least significant bits of a binary number counts modulo eight.Therefore the least significant three bits of each new field number mustequal the least significant three bits of the predicted field number topass the sector check.

A second check of data integrity is the range check, a test of themaximum range of stylus movement along the radius of the disc. No morethan 63 grooves are expected to be jumped when encountering worst caseconditions in any mode. Groove numbers are represented by the mostsignificant 15 bits of the field number. The microprocessor substractsthe present groove number from the predicted groove number. If thedifference is greater than the acceptable range of 63 grooves, then thepresent data is considered a "bad read". All other reads are regarded asgood reads and are used to update the predicted field number. Afterfifteen consecutive bad reads, the system re-enters the ACQUIRE mode.The presence of a scan signal in certain modes, as shown in FIG. 10,will also cause a transition to ACQUIRE mode.

When going from ACQUIRE mode to PLAY mode the microprocessor sets thebad read count to thirteen. This means that when entering PLAY mode fromACQUIRE mode, one of the next two fields must supply a good read or thebad read count will reach fifteen causing a return to ACQUIRE mode.

If the pause button is pressed during PLAY mode, the system enters PAUSEmode. In this mode the stylus is off the record and is held in its thenradial position over the record. When the pause button is released,PAUSE LATCHED mode is entered and held. Pressing the pause button againreleases the PAUSE LATCHED mode, causing a transition to ACQUIRE mode.END mode is entered from PLAY mode when band number sixty three isdetected.

FIG. 11 is a flow chart of the program executed by the microprocessor.The microprocessor hardware includes one interrupt line and aprogrammable timer. A commercially available microprocessor suitable forthe present system is the Fairchild Semiconductor model F8.

The microprocessor uses the timer to control the window in time that theinformation buffer searches for data. This "data window" isapproximately twelve horizontal lines wide and is centered about theexpected data. When no data is found, the timer maintains internalprogram synchronization to one field time interval.

The microprocessor interrupt is coupled to the status signal onconductor 75 (FIG. 4). Interrupts are enabled only in ACQUIRE mode whenthe system continuously searches for data. The program is interruptedwhen a digital message is received. The interrupt service routine (notshown) sets an interrupt flag if the error code check indicatesvalidity. Thereafter, in PLAY mode, the programmable timer is used toindicate the estimated time of arrival of the next digital message.

Switch inputs (load, scan, and pause) are conditioned to prevent switchbounce from causing undesired player response. The microprocessorprogram includes logic to debounce switch inputs. Debounced switchvalues are stored in memory. A separate debounce count is maintained foreach switch. To check debounce 154 the switches are sampled and comparedto the stored switch value. If the sampled state and the stored stateare the same, the debounce count for that switch is set to zero. Switchstates are sampled as often as possible. Each field, (every 16milliseconds for NTSC), all debounce counts are incrementedunconditionally. If the resulting debounce count is equal to or greaterthan 2, the stored state is updated to the new (debounced) value. Thenew switch state is then acted upon.

The first programed step (FIG. 11), after power is turned on, isinitialization 150 of all program parameters. The timer is set to timeout one video field. Mode is set to LOAD.

The next step 152 is a program to carry out the state transition logicrepresented in FIG. 10. Debounce counts are normally incremented at thistime, and tested to determine whether a new switch state is fullydebounced.

After the mode selection logic 152, the program enters a tight loop 153to (1) sample switches setting debounce counts to zero, if required 154,and (2) check if the timer is close to time out 155, and (3) check ifthe interrupt flag has been set 156.

If the interrupt flag is set 156, the program transfers data, 157a, fromthe information buffer and sets the timer, 157b, to time out a new fieldinterval. When the interrupt service routine sets the interrupt flag,the contents of the timer are saved in memory. The program now uses thepreviously stored timer contents to set the timer, 157b, with acorrected value predicting the approximate time of occurrence of thenext digital message. As previously noted, even though the datarepresents the first good read in ACQUIRE mode, the bad read count isset, 157c, to 13.

If the interrupt flag is not set, the program branches as the timer getsclose to time out, 155. If the machine is not in PLAY mode 159, then thetimer is set to time out another field interval, 158. If the machine isin PLAY mode, 159, then a number of time critical tasks, 160, areperformed. The data window is opened, 160a, (by setting control signalon conductor 71 in FIGS. 4 and 8 to a logical one) approximately sixhorizontal lines before the expected data. Received data is read andchecked as previously described. After data is received, or if no datais received, the data window is closed. Timer content, which representsthe actual time of arrival of the digital message, is used as acorrection factor to set the timer again, 160b. The timer is thereforeset to center the next data window over the predicted time of arrival ofthe next digital message based on the actual time of arrival of thepresent digital message.

Expected field number is updated, 160c, band number is checked for start(band 0) and end of play (band sixty three), and the bad read count isincremented, 160g, for a bad read. For valid field data in the programviewing material, time is calculated and displayed, 160f. If valid fielddata indicates that the stylus has skipped backward, the stylus kickermeans is activated, 160e, and ACQUIRE mode is entered. Also, if the badread count reaches 15, ACQUIRE mode is directly entered. Throughout thetime utilized for critical tasks 160, the switch debounce check routineis repeated periodically so that switches are tested as often as isfeasible. The program unconditionally returns through the mode selectionlogic 152 to the tight loop 153 and waits for the timer test 155, or theinterrupt check 156, to indicate the arrival of the next digital messge.

The timer may be set by loading the timer directly via programmedinstructions. However, rather than use a sequence of instructions, it isbest to "set" the timer by establishing a location in memory (a mark)which corresponds to a time out condition of the timer. The timer, then,is free running. Time out, or closeness to the time out, is detected bycomparing the contents of the timer to the mark set in memory. The nextdesired time out condition is set by adding the next desired timeinterval to the previous timer contents and storing the result inmemory. The timer is thus "set" each time valid data is received, or ifno data is received within the data window, by setting a new mark inmemory corresponding to the next time out condition.

The programmable timer in the microprocessor used in the presentembodiment is conditioned by the program to divide cycles of the input1.53 MHz clock by a factor of 200. The timer thus counts once for every200 cycles of the 1.53 MHz clock. One vertical field (1/60 second forNTSC) is then approximately 128 timer counts. One may alternatively usea timer which counts a different multiple of the 1.53 MHz clock, or onethat uses a timing source independent of the video signal.

The data window is made wide enough to allow for several sources oftiming error. Timer uncertainty due to the finite resolution of thetimer is equal to one least significant bit, which corresponds to twohorizontal lines. Accumulated drift error, because 128 timer counts isnot exactly one vertical field, is somewhat less than one line after 16consecutive fields in which no valid message is found. It is noted thatsince the 1.53 MHz color subcarrier clock is an odd multiple of one halfthe line frequency, a timer which counts a corresponding multiple of thecolor subcarrier clock would have zero drift rate. In the particularembodiment described herein, program uncertainty in determining the timeof arrival of the data, is approximately 97 microseconds, or about 1.5lines. Finally, because alternate fields are interlaced, the time fromone digital message to the next is either 262 lines or 263 lines,depending on whether the present field is odd or even. Although theprogram could keep track of odd and even fields, it is simpler to justwiden the data window by one additional line. Combining the abovefactors, it can be shown that a data window extending three timer counts(about 6 lines) both before and after the start of expected data isadquate to allow for worst case timing conditions.

TRACK ERROR CORRECTION

As mentioned earlier, field number information may be used to detectlocked grooves. If the new field number (after sector and range check)is less than the expected field number, then the stylus has skippedbackward and is repeating the tracking of a previously playedconvolution(s), i.e. a locked groove has been encountered. If the newfield number is greater than the expected field number, the stylus hasskipped forward, i.e. toward record center. In the present application,skipped grooves are ignored; if the new field number is greater (butstill passes sector and range check) then the expected field is updatedto the new field. In certain other applications, such as whether thevideo disc is used to record digital information on many horizontallines, it may be necessary to detect and correct skipped grooves aswell. However, for the present video application, a locked groove iscorrected by operating a stylus "kicker" until the stylus is returned tothe expected track. Eventually, the stylus will be advanced past thelocked groove defect.

In a more general sense, the use of field number information inaccordance with the present disclosure provides an accurate means fordetecting general tracking errors. In any video disc system havingspiral or circular tracks, including optical and grooveless systems,tracking errors due to defects and contaminants are always possible. Thepresent system provides a means for detecting and correcting suchtracking errors in a video disc player. For positive tracking, abi-directional kicker means is provided for moving the pickup backwardor forward in the program material. Thus, when a tracking error isdetected, whether a skipped track or a locked track, the pickup is movedin such direction so as to correct the tracking error. Although theregular pickup servo could be used for track error correction purposes,a separate kicker, or pickup repositioning means, is preferble. Theregular servo is generally adapted for stable tracking of the spiralsignal track, and may not have the proper characteristics to respond toabrupt tracking errors. A separate kicker, on the other hand, can bespecifically adapted to provide the fast response needed to correcttracking errors. A specific example of a kicker suitable for use withthe disclosed apparatus may be found in U.S. patent application Ser. No.39,358, for E. Simshauser entitled "TRACK SKIPPER APPARATUS FOR VIDEODISC PLAYER", filed May 15, 1979 and assigned to the assignee of thepresent invention.

Several control algorithms are possible. The pickup apparatus may bereturned directly to the correct track by producing stylus motionproportional to the magnitude of the detected tracking error. Or, akicker may be operated in response to a series of pulses, wherein thenumber of pulses is proportional to the magnitude of the detectedtracking error. The pickup is moved a given number of tracks per pulseuntil the stylus is back on the expected track. For certain applications(e.g. retrieving digital data stored on the video disc medium) it may bedesirable to return the pickup to the point of departure and attempt asecond read, rather than return the pickup to the expected track. In anyevent, it is seen that by the use of a kicker and suitable controllogic, successful tracking can be obtained even though the video disccontains defects or contaminants which would otherwise causeunacceptable tracking errors.

In a digital track correction system, security against undetected dataerrors is particularly important to prevent noisy signals from advancingor retarding the pickup unnecessarily. The present data system reducesthe probability of an undetected read error to a negligible level.

To a rough approximation, one can estimate the probability that a randomdigital input will appear to the data system as a valid messagecontaining a non-sequential field number, thereby actuating the styluskicker. The random probability of a good start code is 1 in 2¹³. Therandom probability of a good error code is also 1 in 2¹³. The randomprobability of a good field number is calculated as follows. Fieldnumbers contain 18 bits. Since there are eight sectors on the disc ofthe system under consideration, the least significant 3 bits of eachfield number indicate the sector number, which must match the expectedsector number. The remaining fifteen bits, which represent groovenumber, can vary over the allowable range (plus or minus 63 grooves).Therefore only 126 out of 2¹⁸ random field numbers will pass the sectorand range checks. Combining all safeguards, the probability of anundetected error is 126 in 2⁴⁴.

The above estimate is based on an assumption of a truly random input,and it does not take into account several factors which further reducethe probability of an undetected error.

For example, on a video disc track, burst noise, where the erroneousbits are adjacent to each other, is more likely than other types ofnoise. As previously noted, the particular error code chosen detects allsingle burst errors up to 13 bits, and a high percentage of all longerbursts as well. Also, as previously explained, the choice of a non-zeroremainder for the error check code (a coset code) further reduces theprobability of an undetected error. Furthermore, the particular startcode chosen, a Barker code, reduces the probability that noise willcause a false start code detection.

The disclosed data system, as applied to the video disc system, resultsin a rate of undetected errors which is relatively low and false alarmswhich would otherwise cause unnecessary stylus movement aresignificantly reduced. The data security provided by the disclosedsystem improves the stability of many player functions, such as displayof program playing time, which depend on recorded digital data forproper operation.

What is claimed is:
 1. In a video disc player for playing a video dischaving signal tracks, said signal tracks recording a carrier modulatedwith a video signal, said video signal including signals representing aplurality of digital numbers in predetermined orderly sequence encodedtherein, said video disc player including a signal pickup means forsensing said recorded video signal, a system for reducing trackingerrors comprising:means coupled to said signal pickup means for decodingsaid recorded digital numbers; means for storing selected decodeddigital numbers and incrementing said selected number on the occurrenceof subsequent digital numbers; means responsive to said decoded digitalnumbers and said incremented number for providing an error indicationsignal when consecutively decoded digital numbers disagree with theincremented stored digital number, wherein said error indication signalis indicative of a detected tracking error; and means responsive to saiderror indication signal for moving said signal pickup means to othersignal tracks recording said video signal in such direction as toovercome said detected tracking error.
 2. In a video disc player forplaying a video disc having signal tracks, said signal tracks recordinga carrier modulated with a video signal, said video signal includingsignals representing a plurality of digital numbers in predeterminedorderly sequence encoded therein, said video disc player including asignal pickup means for sensing said recorded video signal, a system forcorrecting locked track errors comprising:means coupled to said pickupapparatus for decoding said recorded digital numbers; means coupled tosaid decoding means for storing said decoded recorded digital numbers;means responsive to said decoded digital numbers and the stored decodeddigital numbers for providing an error indication signal when at leasttwo consecutively decoded digital numbers are in the reverse order ofsaid predetermined orderly sequence, said error indication signal beingindicative of a locked track error; and means responsive to said errorindication signal for moving said signal pickup means to advanced signaltracks recorded on said video disc so as to correct said detected lockedtrack error.
 3. A system for reducing tracking error in a video discplayer as set forth in claims 1 or 2 wherein said means for moving saidsignal pickup means comprises:a kicker means operatively associated withsaid pickup means for producing movement of said pickup meansessentially orthogonally to said signal tracks, in response to each ofsaid error indication signals.
 4. In a video disc player for playing avideo disc having substantially spiral groove on the surface thereof,said spiral groove containing signal tracks recording a carriermodulated with a video signal, said video signal including signalsrepresenting a plurality of digital numbers in predetermined orderlysequence encoded therein, said video disc player including a pickupapparatus for sensing said recorded video signal, said pickup apparatusfurther including a groove engaging stylus, a system for reducingtracking errors comprising:means coupled to said pickup apparatus fordecoding said recorded digital numbers; means coupled to said decodingmeans for storing said decoded recorded digital numbers; meansresponsive to said decoded digital numbers and said stored decodeddigital numbers for providing an error indication signal whenconsecutively decoded digital numbers depart from said predeterminedorderly sequence, wherein said error indication signal is indicative ofa detected tracking error; and kicker means responsive to said errorindication signal for moving said pickup stylus to another grooveconvolution of said video disc, as determined by the magnitude of suchdetected tracking error and in such direction as to reduce said detectedtracking error.
 5. A system for reducing tracking errors in a video discplayer as set forth in claim 4 wherein said means for providing an errorindication signal includes:means for determining the magnitude of adetected tracking error including means for providing said errorindication signal as a signal proportional to the magnitude of saiddetected tracking error.
 6. A system for reducing tracking error in avideo disc player, as set forth in claim 4 wherein said means forproviding an error indication signal includes:means for determining themagnitude of said detected tracking error including means for providingsaid error indication signal as a series of signal pulses wherein thenumber of said signal pulses is directly proportional to the magnitudeof said detected tracking error; and said kicker means is of a typeresponsive to said series of signal pulses for moving said stylus adistance equivalent to one adjacent groove convolution for each of saidsignal pulses in such direction as to reduce said detected trackingerror.
 7. In a video disc player for playing a video disc having aspiral groove on the surface thereof, said groove containing signaltracks recording a carrier modulated with a video signal, said videosignal including a digital signal on one horizontal line of each fieldthereof, said digital signal encoding a digital field numbercorresponding to the respective video field containing said digitalsignal, said video disc player including a pickup apparatus for sensingsaid recorded video signal, said pickup apparatus further including agroove engaging stylus, a system for correcting a locked groove errorcomprising:means coupled to said pickup apparatus for decoding saidrecorded digital numbers; means coupled to said decoding means forstoring the decoded recorded digital numbers; means responsive to saidstored decoded recorded digital numbers and said decoded digital numbersfor providing an error indication signal when at least two consecutivelydecoded digital numbers are in reverse order as compared to saidpredetermined orderly sequence, said error indication signal beingindicative of a locked groove; and kicker means responsive to said errorindication for moving said pickup stylus to an advanced grooveconvolution relative to the location of said locked groove so as tocorrect said detected locked groove error.
 8. A system for correcting alocked groove error in a video disc player as set forth in claim 7,wherein said means includes:means for determining the magnitude of saiddetected locked groove error including means for providing said errorindication signal as a signal proportional to the magnitude of saiddetected locked groove error; and wherein said kicker means is of a typeresponsive to said error indication signal for advancing said stylusover a number of groove convolutions as determined by the magnitude ofsaid detected tracking error so as to correct said detected lockedgroove error.
 9. A system for correcting a locked groove error in avideo disc player as set forth in claim 7, wherein said means forproviding an error indication signal includes:means for determining themagnitude of said detected locked groove error including means forproviding said error indication signal as a series of signal pulseswherein the number of said signal pulses is directly proportional to themagnitude of said detected locked groove error; and wherein said kickermeans is of a type responsive to said series of signal pulses foradvancing said stylus by one adjacent groove convolution for each ofsaid pulses as to correct said detected locked groove error.
 10. In avideo disc player as set forth in claim 7, a means for displaying saidsignal pickup groove position comprising:further means responsive tosaid decoded digital numbers for computing program playing time bydividing each digital field number by a constant equal to the number offields per unit of playing time; and display means responsive to saidfurther means for displaying said program playing time.
 11. In a videodisc player for playing a video disc having signal tracks recording acarrier modulated with a video signal, said video signal including adigital signal on one horizontal line of each video field thereof, saiddigital signal representing a digital field number corresponding to therespective video field containing said digital signal, wherein saiddigital field numbers have ascending consecutive values recorded insucceeding video fields of said video disc recorded signal, said videodisc player including a pickup apparatus for sensing said recorded videosignal, a program position display apparatus comprising:means coupled tosaid pickup apparatus for decoding said recorded digital numbers;further means responsive to said decoded digital numbers for computingprogram playing time by dividing each digital field number by a constantequal to the number of fields per unit of playing time; and displaymeans responsive to said computed program playing time from said furthermeans for displaying said program playing time.